Water deficit and drought are the main factors that limit crop production and productivity, and are a major threat to food security worldwide. Plant varieties can be bred to be more productive per unit of water supplied, i.e., higher water-use efficiency (WUE). However, our lack of knowledge of the genetic mechanisms underlying WUE has hindered the improvement of this trait. Water-use efficiency may be impacted by a number of factors, including stomatal conductance, which is partially regulated by hydraulic conductivity. Hydraulic conductivity increases rapidly with greater xylem vessel diameter because flow is proportional to the fourth power of conduit diameter. Genes that regulate meristematic cell differentiation into vessels were unknown until recently, but their manipulation could increase hydraulic conductivity and, consequently, the photosynthetic rate and plant productivity.